Chronic dexmedetomidine, a selective alpha 2-agonist, decreases serotonin but not noradrenaline turnover in rat brainstem nuclei.

The effects of a chronic treatment with dexmedetomidine, the active d-isomer of a selective alpha 2-agonist, medetomidine, on the metabolism and turnover of catecholamines and indoleamines in discrete rat brain stem and mesencephalic nuclei were investigated. Chronic dexmedetomidine (3 or 10 micrograms/kg per h for 14 days, delivered by osmotic minipumps) did not change the basal concentrations of noradrenaline, dopamine or 3,4-dihydroxyphenylacetic acid, or the rates of accumulation of 3,4-dihydroxyphenylalanine after inhibition of decarboxylase by NSD 1015. In contrast, the concentrations of 5-hydroxyindoleacetic acid (5-HIAA) and/or 5-HIAA/5-HT were significantly reduced in the nucleus tractus solitarii, raphe dorsalis, locus coeruleus, A1-C1 area, A9 and A10 areas after 10 micrograms/kg per h of dexmedetomidine. The accumulation of 5-hydroxytryptophan was significantly reduced in the locus coeruleus and nucleus tractus solitarii. It is concluded that chronic dexmedetomidine treatment significantly decreases the turnover of 5-hydroxytryptamine in discrete brainstem and mesencephalic nuclei, while the rate of synthesis of noradrenaline in the same areas is not changed.